Character mistake: After Macer shoots the shotgun man during the house raid, she checks his pulse without removing the weapon from his hands first.
Character mistake: Just before coming back to the U.S. at the border, Forsing takes his hand off the trigger to unlock the truck door. For anyone properly trained the trigger hand will always stay on the weapon - the forehand would be used for everything else.
Answer: They haven't crossed the border yet, and the gang members were there to kill them. They had no intention of actually crossing the border.